This invention relates generally to balun and impedance transformers and more specifically to broadband balun and impedance transformers for push-pull amplifiers.
Broadband push-pull power amplifiers have the advantage of higher output power (twice that of a single-ended amplifier) with even harmonic cancellation and high efficiency. For push-pull power amplifiers to work properly, a broadband balun with excellent amplitude and phase balance is needed. Additionally, for practical amplifier designs, an impedance transformer that can transform low impedances (e.g. 3 to 4 ohm) to an external load impedance (e.g. 50 ohm) over, for example, a 700 MHz to 2.4 GHz frequency range is required. The balun/transformer also needs to provide DC biasing to the push-pull amplifier.
Many prior art power amplifier applications address the need for a transformer but not the need for a balun, while many mixer applications address the design of baluns that cannot be used in conjunction with an impedance transformer. A balun is used mainly to connect a balanced source to an unbalanced load, or vice versa, and a balun typically provides no impedance transforming function.
Accordingly, it would be desirable to have a combination of impedance transformer and balun to create a broadband impedance transformation while at the same time being able to provide balanced differential ports for push-pull amplifier applications.